OFDM is an orthogonal frequency division multiplexing technology. In fact, the OFDM is one type of MCM (multi-carrier modulation). Its main idea is to divide a channel into several orthogonal subcarriers, convert a high-speed data signal into parallel low-speed data subflows, and modulate them onto the subcarriers for transmission. Orthogonal signals may be separated at a receiver end by using a related technology, thereby reducing mutual ISI (inter-symbol interference) between the subcarriers. Other features such as high bandwidth usage and simple implementation enable the OFDM to be more widely applied in the field of radio communications. For example, a WLAN (wireless local area network) system, a WiMAX system based on orthogonal frequency division multiple access (OFDMA), and a fourth-generation mobile telecommunications system (4G) are all systems based on the OFDM technology.
An OSD (overlapped spectrum duplex) technology refers to a technology in which a spectrum overlapping technology is used in both an uplink and a downlink to send and receive a signal. The OSD technology enables uplink and downlink signals to multiplex all bands completely at the same time, which is expected to double spectrum efficiency compared with a traditional FDD (frequency division duplex) or TDD (time division duplex) mode. However, as a transmission distance becomes shorter, a channel delay becomes smaller, and a signal sent by a near-end device and a signal received by a far-end device are almost completely orthogonal to each other in terms of time. However, in an OFDM system, subcarrier signals of orthogonal frequency division signals sent by the near end and the far end are not completely synchronous. As a result, an echo OFDM signal that has passed through a hybrid coil is not synchronous with a far-end OFDM signal, which causes severe interference to the received signal.